Capacitors, such as flow-through capacitors may be used for many purposes, including chromatography, water purification, separation of blood components, and other circumstances in which separation of an ionic constituent from a liquid is desired, such as recovery of precious metals or other chemicals from a liquid stream. In a flow-through capacitor, a liquid, e.g., salt water, flows over the electrode to remove ionic constituents from the liquid. In an electrochemical double layer capacitor, the liquid mediates the movement of ions to and from the electrode. The use of flow-through capacitors for purification of water, for example deionization of salt water, is also known as “capacitive deionization.”
Flow-through capacitors generally include one or more pairs of spaced apart electrodes—a cathode and an anode—with current collectors or backing layers provided that are generally adjacent to or very near the electrodes. There is also a flow path for a liquid to travel through the flow-through capacitor and contact the current collectors and electrodes. Current collectors are electrically conductive and communicate with the electrodes to carry an electrical charge.
Flow-through capacitors have been described in several patents, including U.S. Pat. No. 5,779,891, to Andelman, the disclosure of which is incorporated herein by reference. Flow-through capacitors may have any of several different configurations, such as a planar configuration illustrated in U.S. Pat. No. 5,538,611 or spiral-wound, crescent-pleated, or hexagonal-shaped configurations, as illustrated in U.S. Pat. No. 5,779,891. The selection of the shape of the flow-through capacitor is dependent upon the preferences of the user and may be of any conventional shape.
A conventional planar flow-through capacitor 9 is illustrated in FIG. 1. A separator 1 is provided that separates the flow-through capacitor 9 into a positive charge side and a negative charge side. Conductive high surface area material 2 is located adjacent to the separator 1 and is adjacent current collector 3. The conductive high surface area material 2 is porous. In the conventional flow-through capacitor 9 illustrated in FIG. 1, the conductive high surface area material 2 is held in place by the configuration of the flow-through capacitor 9, in which retaining plates 4 are fastened together (see below), compressing the elements of the capacitor there between in a “sandwich” fashion. Other flow-through capacitor configurations similarly have the conductive high surface area material 2 held against the current collector 3 by compressive force. Regardless of the specific configuration, the conductive high surface area material 2 also may be held against the current collector 3 by mechanical fastening, attachment, adhesion, or in any other conventional manner.
The current collectors 3 have terminals 3a extending therefrom and may have flow-through orifices 3b. The current collectors 3 may be made from a wide variety of materials, including copper, aluminum, and carbon boards.
The flow-through capacitor 9 is provided with retaining plates 4 that support the flow-through capacitor 9 and provide rigidity while enclosing the elements of the flow-through capacitor 9. Bolt holes 8 are provided to secure the retaining plates 4 to each other. The flow-through capacitor also has spacers 5 that provide an offset of the current collectors 3 from the retaining plates 4.
The retaining plates 4 are provided with a fluid inlet 6 for fluid to enter the flow-through capacitor 9 and a fluid outlet 7 for fluid to exit the flow-through capacitor 9 after it has contacted the conductive high surface area material 2.
Flow-through capacitors generally operate by applying a voltage across the electrodes, with the charge generated transferred to the current collectors. The current collectors, one with a positive charge and one with a negative charge, are contacted by the liquid stream and attract oppositely charged ions out of the liquid stream to the surface of the current collector. The efficiency of the flow-through capacitor is thus dependent on the amount of surface area available for contact with the liquid to attract ions therefrom.
Flow-through capacitors may be arranged in series or parallel, depending on the needs of the particular circumstances faced by a user, and they come in many different shapes and sizes.
When it is desired to release the collected ions, such as when the efficiency of the flow-through capacitor is reduced to a predetermined level, the electrodes may be shorted to ground or otherwise have the charges removed from the current collectors, causing the attracted ions to leave the surface of the collectors and return to the liquid stream. Thus the liquid stream at this point will have a higher concentration of the ions that were previously removed from the liquid passing through the flow-through capacitor. The liquid stream may be discarded or may be further treated to recover the ions that were released into the liquid stream. In such way, recovery of the ionic material may be accomplished.
This predetermined efficiency level may be measured, for example, by the conductivity of the liquid exiting the capacitor or by any other measure convenient for the user. The user may elect not to use a predetermined efficiency, but to discharge the ionic material after a preselected time interval or by any other method selected by the user for determining when to discharge the collected ionic material.
The electrically conductive backing materials in many flow-through capacitors are made from carbon cloth or flexible graphite. U.S. Pat. No. 6,410,128 B1, to Calarco, et al., the disclosure of which is incorporated herein by reference, describes the construction and use of flexible graphite as an electrode and backing material in flow-through capacitors.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and, thus, exhibit or possess many properties that are highly directional, e.g., thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g., webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g., roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g., web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e., the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e., along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.